Well, yes, browsers are inconsistent. Surprise, surprise.
In particular, there are NO STANDARDS as to how browsers size fonts, except when you size them in pixels. Even then, there is no guarantee that the font characters are individually the same size (because they may not be the same font at all) and so, rendering of text, in particular, should be expected to vary from one browser to the next.
I don't think that JQM has ever claimed to produce pixel-perfect identical results across browsers. This is largely up to you in how you choose your fonts, how you size them, etc.